Optical fiber communication systems are lightwave systems that employ optical fibers to transmit information. Optical fiber communications systems include optical transmitters, optical receivers, and transmission media that propagate information between the optical transmitters and the optical receivers. An optical transmitter for an optical fiber communication system includes an optical source, such as a semiconductor laser, that generates an optical signal and an optical modulator that modulates the optical signal with data or voice information. The modulated optical signal is transmitted through a transmission media, such as an optical fiber, to an optical receiver. The optical receiver detects the transmitted optical signal and processes the optical signal into an electronic waveform that contains the data or voice information.
Optical fiber communication systems are now widely deployed. Recently, relatively new communications services, such as the Internet, high-speed data links, video services, wireless services and CATV, have resulted in a dramatic increase in the need for higher information data rates. The aggregate data throughput rate of a communication system can be increased either by increasing the bandwidth of an individual data channel or by increasing the number of data channels.
In addition, many optical fiber communication systems today are being built to transmit data over long distances with high data rates. Moreover, such systems are currently being built to transmit data and voice information over these longer distances without employing repeaters in order to reduce the capital and operating costs associated with transmitting data. In order to achieve these higher data rates and longer transmission distances, optical signals having relatively narrow line widths must be transmitted at relatively high intensity.
The noise detected at the receiver increases as the bandwidth of an individual channel is increased. The amount of optical power in the transmitted carrier signal must be increased to maintain a sufficient signal-to-noise ratio at the receiver as the bandwidth of an individual channel is increased. However, the propagation distance that can be achieved using a carrier signal with increased power in a narrow line width is severely limited by a physical effect known as stimulated Brillouin scattering (SBS).
Stimulated Brillouin scattering is a stimulated scattering process that converts a forward traveling optical wave into a backward traveling optical wave that is shifted in frequency relative to the forward traveling optical wave. Backward scattering occurs within optical fibers because of coupling between acoustic phonons created by vibrational excitation of acoustic modes in the optical fiber material itself and by the incident photons of the optical signal.
The acoustic phonons and photons generate transient gratings that produce backward scattering and frequency shifting of the incident optical signals. The frequency shifting is typically between about 10–100 MHz for commonly used optical communication fibers. Stimulated Brillouin scattering also causes multiple frequency shifts. In addition, SBS can permanently damage the optical fiber if the optical propagating power is sufficiently high.
The transmission quality of optical signals having relatively high intensity and narrow line width can be improved by reducing the effects of SBS. The increase in the transmission quality can allow data and voice service providers to increase the optical signal power level, and therefore, increase the possible propagation distance of their communication links between repeaters. Consequently, reducing the effects of SBS can reduce the cost per bit to transmit data and voice information.